1. Field of the Invention
The invention relates generally to the field of fluid dynamic bearings, and more specifically the present invention relates to apparatus for preventing the loss of fluid from a fluid dynamic bearing motor.
2. Description of the Related Art
Disc drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric tracks of a magnetic disc medium, the actual information being stored in the forward magnetic transitions within the medium. The discs themselves are rotatably mounted on a spindle, while the information is accessed by read/write has generally located on a pivoting arm which moves radially over the surface of the rotating disc. The read/write heads or transducers must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information.
During operation, the discs are rotated at very high speeds within an enclosed housing using an electric motor generally located inside the hub or below the discs. Such known spindle motors typically have a spindle mounted by two ball bearings systems to a motor shaft disposed in the center of the hub. The bearings are spaced apart, with one located near the top of the spindle and the other spaced a distance away. These bearings allow support the spindle or hub about the shaft, allow for a stable rotational relative movement between the shaft and the spindle or hub while maintaining accurate alignment of the spindle and shaft. The bearings themselves are normally lubricated by highly refined grease or oil.
The conventional ball bearing system described above is prone to several shortcomings. First is the problem of vibration generated by the balls rolling on the bearing raceways. Ball bearings used in hard disc drive spindles one of the conditions that generally guarantee physical contact between raceways and balls, this in spite of the lubrication provided by the bearing oil or grease. Hence, bearing balls running on the generally even and smooth, but microscopically uneven and rough raceways, transmit the rough surface structure as well as their imperfections in sphericity in the_ vibration of the rotating disc. This vibration results in misalignment between the data tracks and the read/write transducer. This source of vibration limits, therefore, the data track density and the overall performance of the disc drive system. This vibration results in misalignment between the data tracks and the read/write transducer. This source of vibration limits therefore the data track density and the overall performance of the disc drive system.
Further, mechanical bearings are not always scalable to smaller dimensions. This is a significant drawback, since the tendency in the disc drive industry has been to continually shrink the physical dimensions of the disc drive unit.
As an alternative to conventional ball bearing spindle systems, much effort has been focused on developing a fluid dynamic bearing. In these types of systems lubricating fluid, either gas or liquid, functions as the actual bearing surface between a stationary shaft aft supported from the base of the housing, and the rotating spindle or hub. Liquid lubricants comprising oil, more complex ferromagnetic fluids, or other lubricants have been utilized in such fluid dynamic bearings. The reason for the popularity of the use of such fluids is the elimination of the vibrations caused by mechanical contact in a ball bearing system, and the ability to scale the fluid dynamic bearing to smaller and smaller sizes.
An issue which requires constant consideration in the design of a fluid dynamic bearing is preventing the loss of fluid from the bearing. Such loss can occur either due to evaporation, or to a high level shock to the bearing. One of the more popular types of fluid dynamic bearings is a conical bearing, wherein a general conical shape piece is attached or supported at or near the ends of the shaft to support a spindle or hub for rotation. The fluid bearing is provided between an angled surface on the conical piece and a facing surface on the spindle or hub; and a reservoir is provided at the outer end of the conical piece, defined by a seal shield which is supported from the hub or sleeve and extends generally radially toward the shaft. The inner surface of the seal shield and outer surface of the conical piece are designed to define both the reservoir and a centrifugal capillary seal at the outer end of the bearing. This seal is designed to utilize capillary attractive force to retain the oil or fluid within the reservoir during non-operating shock and vibration events. The shock retention capability of this seal is approximately 250Gs. When the shock levels exceed 250Gs, oil can leave the main body of the reservoir, and become trapped in the space which must be provided between the outer surface of the shaft and the end surface of the shield, called the annulus. This in itself is not necessarily a problem; but at shock levels somewhere in access of 250-300Gs, but typically below 500Gs, oil trapped in the annulus can be ejected and contaminate the disc drive. As the demands on shock resistance are increased, a greater level of ability to retain all the fluid in the fluid dynamic bearing at higher shock levels is increasing. Therefore, a robust, easy to assemble solution to the problem of potential fluid loss from a conical fluid dynamic bearing, one that also does not require any significant redesign of the known conical bearing and centrifugal capillary seal is highly desirable.
It is an object of the present invention to provide a hydrodynamic bearing design with increased resistance to loss of the lubricating fluid.
It is a further objective of the invention to provide an improved design for a conical bearing which utilizes a centrifugal capillary seal at the outer end for fluid retention.
It is a further objective of the invention to provide apparatus which minimizes the amount of oil which can reach and become trapped in the radial gap.
It is a further objective of the invention to provide a modified design of the capillary seal which provides for active pumping to remove oil which is lodged in the annulus. In another objective of the present invention is to provide a modified design of fluid bearing and the seal which makes it more difficult for oil, having reached the radial gap or annulus, to be expelled from that annulus into the interior of the disc drive.
In summary, according to the present invention a conical bearing is provided having a seal shield having an angle supported from the hub or sleeve which surrounds the shaft, and extending at an angle toward the outer surface of the shaft and spaced slightly away from the upper angular surface of the cone. As the cone and seal shield rotate relative to one another, fluid is drawn toward the lower inner region of the reservoir. However, due to shock or the like, some fluid may reach the radial gap between the end of the shield and the outer surface of the shaft, therefore, a ring is either incorporated into the upper end of the cone or pressed against the axial outer end of the cone, defining an axial gap which is smaller than the radial gap. In a preferred form of the invention, the ratio is about 5:1.
When oil is introduced to either of the two gaps (typically by shock), it transfers or typically comes to rest in the axial gap, since the capillary attractive force increases as the gap size decreases; therefore, with the axial gap being smaller than the radial gap the oil will tend to transfer to the axial gap. As the seal is spun up as the shield and cone rotate relative to one another, centrifugal force acting on the oil in the axial gap transfers the oil radially away from the radial gap and into the reservoir defined between the shield and the cone. This transfer typically happens in two stages. First the oil in the radial gap is transferred into the axial gap, typically in the first few seconds of operation. The oil remaining in the axial gap then transfers into the reservoir volume due to centrifugal pumping.
A simplified method of positioning the ring comprises placing the ring on the shaft between the cone and the shield. The ring is put in final position relative to the shield by pressing on the outer end of the shield with a known force, deflecting the shield a calculated distance and moving the ring accordingly.
Other features and advantages of the invention will be apparent to a person of skill in the art who studies the teachings of a exemplary embodiment given in detail below when conjunction with the accompanying drawings.